When I had a case with a restrictive front "filter" I removed it. I also removed the rear "grill." That was the case in item 2 of my sig. Now, the point of that was to show what happens when you remove such obstructions. The increase in airflow it massive. For one thing, metal downstream from the heatsink causes air to be bounced off it as if it was a solid wall, unless you put in a noisy exhaust fan or remove the metal.
In a review titled "Noctua A-Series (NF-A14 NF-A15) Review" I did for Vortez.net published on 29-06-13 (crazy European dating scheme) I compared the NF-S12A with the NF-F12 and the NF-P12.
Whenever you mount fans, try to use vibration isolators. I have tried pretty much all of them, and these are the best. Free shipping, too.
Bottom line, though: if you have to case mod, do it before you put your stuff in. And it is better to cut out obstructions rather than to overcome them.